Intra- and intermolecular proton transfer in methyl-2-hydroxynicotinate

• Published in: Journal of luminescence Vol. 110; no. 3; pp. 147 - 163
• Main Authors: Balamurali, M.M., Dogra, S.K.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.11.2004; Elsevier
• Subjects: Absorption spectrum; Atomic and molecular physics; ESIPT; Exact sciences and technology; Fluorescence and phosphorescence spectra; Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion); Fluorescence spectrum; Methyl 2-hydroxynicotinate; Molecular properties and interactions with photons; Physics; Prototropic equilibrium; Theoretical calculations; Fluorescence spectrum; ESIPT; Prototropic equilibrium; Theoretical calculations; Methyl 2-hydroxynicotinate; Absorption spectrum; Stokes shift; Ground states; Fluorescence; Singlet state; Excited states; Electronic structure; Aqueous solutions; Density functional method; Semiempirical method; Quantum theory; Organic compounds
• ISSN: 0022-2313; 1872-7883
• DOI: 10.1016/j.jlumin.2004.05.001

Spectral characteristics of methyl 2-hydroxynicotinate (MEHNA) have been studied using absorption, fluorescence excitation and fluorescence spectroscopy, as well as, using single photon counting nanosecond spectrofluorimeter. MEHNA is present as enol in less polar solvents and keto in polar media. In non-polar solvents, large Stokes shifted fluorescence band is assigned to phototautomer, formed by excited state intramolecular proton transfer (ESIPT), whereas fluorescence is only observed from keto form in polar solvents. In aqueous and polar solvents monocation (MC) is formed by protonating the exo carbonyl oxygen atom in the ground state (S0) and in the first excited singlet state (S1), MC is obtained by protonating carbonyl oxygen atom of the ester. It is formed by ESIPT from exo carbonyl proton to carbonyl oxygen atom of the ester. Dication is formed by protonating both the oxygen atoms. Two kinds of monoanions formed by deprotonating phenolic proton or >N–H proton of keto suggest the presence of enol and keto in aqueous solution. In cyclohexane MC is formed by protonating carbonyl oxygen in both S0 and S1 states. The electronic structure calculations were performed on each species using semi-empirical quantum mechanical AM1 method and density functional theory B3LYP with 6-31G** basis set using Gaussian 98 program, along with potential energy mapping, to characterize the particular species.